1. Field of the Invention
The present invention relates to two-cycle gasoline engine lubricant compositions and to ester base stocks which are components thereof. The two-cycle gasoline engine lubricants according to the invention produce lower amounts of observable smoke in the exhaust emitted as a result of combustion in a two-cycle gasoline engine, require no miscibility-enhancing solvents and are optionally biodegradable.
2. Description of the Related Art
The two-cycle (two stroke) gasoline engine has gained considerable popularity as a power source for such devices as outboard motors, snow mobiles, motorcycles, mopeds and a variety of landscaping equipment, e.g., lawn mowers, chain saws, string trimmers and blowers. The widespread use of two-cycle gasoline engines is due primarily to their simple design and lightweight construction, their ability to provide high power output with quick starts at low temperature and their relatively low cost.
Two-cycle gasoline engines are operated using a mixture of gasoline and a lubricant in prescribed proportions. Because the fuel contains a gasoline-lubricant mixture, large amounts of smoke are generated and emitted in the exhaust. The lubricant must provide satisfactory performance characteristics under severe operating conditions. Lubricants for two-cycle gasoline engines are generally composed of a mineral oil or synthetic base fluid, performance additive(s) and a solvent, ordinarily a relatively low boiling petroleum distillate, to enhance gasoline/lubricant miscibility.
The technologies developed to date for reducing exhaust emissions from four-cycle car and truck gasoline engines have not been successfully adapted to two-cycle gasoline engines. Hence, there is growing public concern over the high levels of hydrocarbon emissions from these small gasoline engines, as hydrocarbons do not readily biodegrade.
The hydrocarbon emissions are a consequence of the basic design of the gasoline engine. Specifically, in the power stroke of a typical two-cycle gasoline engine, air, oil and fuel are drawn into the crank case as the combined charge is compressed in the space above the piston. In the exhaust stroke, the burnt gases are discharged through exhaust ports, and a fresh combustible charge is transferred from the crank case to the space above the piston. Because the exhaust ports open before and close after transfer of the fresh combustible charge occurs, as much as 20% of the fresh charge will be discharged unburnt with the exhaust. Consequently, hydrocarbon emissions far exceed the level of emissions from a comparable four-cycle engine.
Water-cooled outboard motors exhaust directly into the water, giving rise to water pollution, whereas the other devices mentioned above, which are equipped with air-cooled two-cycle gasoline engines, produce emissions that pose a serious air pollution problem. For example, the California Air Research Board has determined that many two-cycle gasoline engines produce up to fifty times the pollution of truck engines per horsepower hour.
Visible smoke emissions in the exhaust from two-cycle gasoline engines has also recently come under increased scrutiny and regulation. In addition, smoky emissions from two-cycle gasoline engines are also becoming a problem from an aesthetic stand point.
The above-noted pollution and smoke problems are exacerbated by the presence of volatile organic solvents in the lubricant. Moreover, some of the solvents used as miscibility enhancers, such as Stoddard solvent, have relatively low flash points, thus creating a potential fire risk, which is of particular concern in connection with the storage and transportation of such products.
Thus, a need has existed for a two-cycle gasoline engine lubricant composition which is formulated so as to prevent pollution by protecting against emission of harmful hydrocarbons into the environment, and to reduce the hazard potential of the solvent-containing lubricants, and to at least minimize smoky emissions. These objectives must be obtained, however, while simultaneously satisfying stringent performance standards, e.g., good lubricity and detergency, particularly on piston rings, superior anti-seizure properties and high gel/floc resistance, and providing optimum miscibility of lubricant and fuel over the applicable range of operating conditions.